1. Field of the Invention
The present invention relates to a driving circuit, and more specifically, to a driving circuit used to drive a light emitting element in an optical transmitter that requires a reduced power supply voltage.
2. Description of the Related Art
An optical transmission system using an optical fiber as a transmission line is used for various applications. An optical transmitter in the optical transmission system comprises a light emitting element, a driving circuit that drives the light emitting element, and an encoding circuit that converts an input data signal into an output signal suitable for transmission to input the driving circuit. In recent years, applications for short-distance transmission system have required a transmission speed of 1 Gbps or more and thus an optical transmitter suitable for an increased transmission speed. On the other hand, a reduced power supply voltage improves reduction of current consumption and heat generation.
The driving circuit driving the light emitting element is based on one of two known methods: a current driving method and a voltage driving method. FIG. 1 shows a configuration of a conventional driving circuit based on the voltage driving method. The driving circuit is composed of a light emitting element 11, a variable resistor R11 that connects an anode terminal of the light emitting element 11 and a power supply (Vcc), and a coupling capacitor C11. A bias current through the light emitting element 11 is determined by the power supply voltage Vcc, a forward voltage at the light emitting element 11, and resistance of the variable resistor R11. An input data signal provides a voltage swing to the light emitting element 11 via the coupling capacitor C11.
The voltage driving method enables the light emitting element to be directly driven by the output from the encoding circuit. Thus, the voltage driving method is more suitable for reducing the size of the optical transmitter than the current driving method, which requires a voltage-current converting circuit.
However, with the reduced power supply voltage Vcc, the conventional voltage-driving-based driving circuit requires the resistance of the variable resistor R11 to be set at a small value in order to obtain a bias current. On the other hand, when the resistance of the variable resistor R11 is reduced, load impedance estimated from the output of the encoding circuit equivalently decreases. This makes it difficult to directly drive the light emitting element 11 on the basis of an output from the encoding circuit or the like.